1. Field of the Invention
The present invention relates to inductor on a silicon substrate, specifically to an inductor device and manufacturing method thereof for enhancing the quality factor of the inductor by disposing trenches on a silicon substrate, and by filling the inside of the trenches with polycrystalline polysilicon not doped with impurities.
2. Description of the Related Art
In the design of monolithic microwave integrated circuits(MMIC), inductor are generally required for impedance matching. The characteristics of an inductor are determined not only by its inductance but also by its quality factor. Recently, it has been made possible to integrate inductor on a substrate. The so-called integrated inductor made it possible to manufacture a silicon MMIC in which both transistors which is active devices and inductor which is passive devices are integrated on the same chip. One of the most difficult problems in implementing these integrated inductor is that the characteristics of the inductor is largely dependant on the resistivity of the silicon substrate on which the inductor resides.
Here, the prior art inductor devices on the silicon substrate and manufacturing method thereof will be described referring to FIGS. 1 and 3.
FIGS. 1a and 1b are a top view and a sectional view of the prior art inductor device for manufacturing MMIC on a silicon substrate respectively.
To manufacture the above mentioned inductor device, first insulating layer 5 is formed on a silicon substrate 3. Then, first metal line 2 of the inductor is formed on the first insulating layer 5. Second insulating layer 6 is formed on the first metal line 2 and the first metal line 2 is connected to second metal line 1 through via 4 in the second insulating layer 6. The quality factor of the inductor is largely affected by capacitance between the metal line of the inductor and the silicon substrate and by the resistance of the silicon substrate.
FIG. 3 is an equivalent circuit of the prior art inductor in which the capacitance by the insulation layers 6 and 5, which are between the second metal line 1 of the inductor and the silicon substrate 3 is denoted by Cox, and the resistance of the silicon substrate 3 is denoted by Rsi. For the quality factor of the inductor to be large, Cox should be small and Rsi should be as large as possible. Thus, insulating layers has been made thick or silicon substrates with high resistivity has been used so far. If the thickness of the insulating layer reaches several to 10 micrometers, however, it is hard to integrate active devices on the same chip. Also, if the silicon substrate with high resistivity is used, it is hard to integrate active devices with the standard processes on the same chip and the process cost will be increased.